DNA as a programmable molecule shows great potential in a wide variety of applications, with the dynamic DNA nanodevices such as DNA motors and walkers holding the most promise in controlled functions for biosensing and nanomedicine. However, a motor or walker that consists of DNA exclusively has not been shown to function within cells because of its susceptibility to endogenous nuclease-mediated degradation. In this contribution, we demonstrate a Y-shaped backbone-rigidified triangular DNA scaffold (YTDS)-directed DNAzyme walker that functions inside living cells to detect microRNAs (miRNAs) with high sensitivity. A novel Y-shaped backbone offers access to geometrically well-defined configurations and increases the rigidity of DNA assemblies, providing a unique, circular, and rigid DNA track within living cells without non-nucleic acid auxiliary materials and enabling the stepwise movement of DNAzyme in an inchworm fashion. This strategy is extended to the construction of larger rigid planar geometric polygon-based DNA walkers, demonstrating unprecedented opportunities to build dynamic DNA nanostructures with precise geometry and versatile functionality.